In particular the invention relates to the automatic control of the tuning circuits of the radio receiver of a pager, operable in RDS/MBS networks, in the course of the production tuning and use of the pager. The pager operates in the standard FM radio frequency range (88 to 108 MHz). A received signal has been modulated into a third harmonic (57 kHz of the 19 kHz subcarrier wave in a standard stereo transmission. The pager scans the frequency range to find a station in the stereo transmission whereof said 57 kHz subcarrier wave is found. The radio receiver of the pager is a super-receiver, and it is narrow-banded compared, e.g., with mobile phones, because of which continuous tuning of the tuning circuits is indispensable to comply to the frequency. Continuous tuning is required of the antenna, the narrow-banded filters, e.g., the filter after the RF amplifier (two circuits), and a first local oscillator, all of which in the present context are usually called tuning circuits.
A principle block diagram of a prior art pager, i.e. of a radio receiver of the super type is shown in FIG. 1. The pager comprises an antenna 1 which receives signals and in which the resonance frequency has to be tuned to the receiving frequency. From the antenna the signal passes to a radio frequency RF amplifier 2 in which the received signal is amplified. From the amplifier 2 the signal travels via radio frequency filters 3a, 3b, wherein the desired frequencies are filtered from the received signal. These filters 3a, 3bare narrow-band filters, that is why it is important that they are tuned to the receiving frequency. The signal output by the radio frequency filters 3a, 3b is mixed in a mixer 5 to produce an intermediate frequency with the aid of a frequency obtained from a local oscillator 4. In order to time the signal to a desired intermediate frequency, it is important that the frequency provided by the local oscillator 4 can be controlled to conform to the frequency of the signal to be received. From the mixer 5, the signal is fed to an intermediate frequency section 6 where, e.g., filters are provided for filtering a desired signal at the intermediate frequency from the frequencies provided by the mixer 5, and then to circuits for processing the intermediate frequency signal.
The FM detected low-frequency signal obtained from the intermediate-frequency signal is fed to a data decoder in which the data received by the pager is processed into a form in which a message can be transmitted, e.g., to the display of the pager. Accordingly, a data decoder may comprise, in addition to the data processing circuits, for instance display control circuits and/or amplifiers to amplify the signal. In addition, the pager comprises a control means 8, preferably a D/A converter, for controlling the frequencies of the tuning circuits 1, 3a, 3b, and 4.
The basic tuning process for the tuning circuits is usually carried out during the production process by controlling manually the inductance or capacitance of the tuning circuit. The transmission of the frequency of the pager, while in operation, is accomplished by controlling the voltages of the capacitance diodes of the tuning circuits with the aid of a common DA converter. The capacitance of the capacitance diode changes as a function of tho voltage and, by changing the capacitance of each tuning circuit, they can be tuned on another frequency (the frequency band of the filter can be shifted, the resonance frequency of the antenna can be changed, and the frequency of the local oscillator can be affected). A second alternative is to use a control capacitor instead of the DA converter and the capacitance diodes, though this is not very practical; instead, the common control of the tuning circuits with one DA converter is an easier, more sophisticated and space saving control procedure.
Thus, for tuning the tuning circuits only one DA converter is conventionally used, with the output voltage of which all tuning circuits are controlled where the capacitance of the capacitance diodes is changed by changing the direct voltage provided by the DA converter. Thus, the tuning of the super-receiver, while in operation, is accomplished by controlling all tuning circuits of the receiver at the same time while only one quantity is being changed (voltage, capacitance, or inductance). This causes a so-called synchronization problem, i.e., the tuning circuits of the receiver are not precisely in alignment with the frequency to be received in the various points of the frequency range to be received because in the course of the production of the receiver the basic tuning must be accomplished manually and part of the tuning circuits are thereafter fixed or they will be tuned only in conjunction with the other tuning circuits. The basic tuning is usually accomplished by means of iteration in the lower and upper ends of the frequency range, whereby the precise tuning is achieved only with regard to part of the frequency range (upper or lower end) whereas external to said part of the range, the tuning may be offset as regards certain tuning circuits, and will thus cause deterioration in the operation as regards the reception sensitivity. Associated with the basic tuning, sufficient attention must be paid to the so-called synchronization between different tuning circuits. This will also lead to a lengthened tuning incident because the correct tuning must be provided by iteration. The procedure is conventional and generally used in the production of all narrow-banded super-receivers.